Method and apparatus for obtaining a stereoscopic thermal image of internal and surface portions of an article

ABSTRACT

An article, the constituent portions of which have differing thermal heating characteristics, is heated. The thermal response on a surface of the article, which is a thermal image of the constituent portions of the article both internal of and on the surface of the article, is detected with two heat detectors, each of which is focused along an optical path and onto a separate one of two discrete incremental areas on the surface. The optical paths of the detectors are cyclically scanned in unison across the surface in a predetermined pattern to focus successive incremental areas of the surface onto the heat detectors, and a separate electrical signal is generated in accordance with the thermal response detected by each heat detector. Each electrical signal is applied to a separate one of two cathode ray tubes, the electron beams of which are scanned in the predetermined pattern, and in synchronism with the scan of the optical paths of the detectors, throughout their entire raster, to generate on a screen of each tube a thermal image of the constituent portions of the article as detected by one of the detectors. After each scan through at least one cycle, the article is rotated an incremental amount about an axis to cyclically change the parallax between the optical paths of the detectors and the second surface. In this manner, the image on one of the screens is a left hand thermal image, and the image on the other screen is a right hand thermal image, of the constituent portions of the article, which together form a stereoscopic thermal image of the constituent portions of the article.

United States Patent 1' Jayachandra et al.

Western Electric Company, Incorporated, New York, NY.

Filed: Aug. 22, 1973 Appl. No.: 390,632

[73] Assignee:

[52] US. Cl 250/353, 250/341, 250/347, 250/475 Int. Cl. G01t 1/00 Field of Search 250/314, 482, 475, 334, 250/316, 348, 349, 340, 341, 342, 351, 347; 350/1, 304

[56] References Cited UNITED STATES PATENTS 1/1973 Barhydt 2/1971 Enborg 250/334 11/1966 Barnes 250/334 3/1951 Whittaker 350/130 Primary ExaminerJames W. Lawrence Assistant Examiner-Harold A. Dixon Attorney, Agent, or Firm-R. A. Lloyd a. PowEP 1R SUPPLY SOURCE Apr. 16, 1974 ABSTRACT An article, the constituent portions of which have differing thermal heating characteristics, is heated. The thermal response on a surface of the article, which is a thermal image of the constituent portions of the article both internal of and on the surface of the article, is detected with two heat detectors, each of which is focused along an optical path and onto a separate one of two discrete incremental areas on the surface. The optical paths of the detectors are cyclically scanned in unison across the surface in a predetermined pattern to focus successive incremental areas of the surface onto the heat detectors, and a separate electrical signal is generated in accordance with the thermal response detected by each heat detector. Each electrical signal is applied to a separate one of two cathode ray tubes, the electron beams of which are scanned in the predetermined pattern, and in synchronism with the scan of the optical paths of the detectors, throughout their entire raster, to generate on a screen of each tube a thermal image of the constituent portions of the article as detected by one of the detectors. After each scan through at least one cycle, the article is rotated an incremental amount about an axis to cyclically change the parallax between the optical paths of the detectors and the second surface. In this manner, the image on one of the screens is a left hand thermal image, and the image on the other screen is a right hand thermal image, of the constituent portions of the article, which together form a stereoscopic thermal image of the constituent portions of the article.

19 Claims, 2 Drawing Figures I24 ,L I68 x LEFLN x-wN c LINE 64 CIRCUIT [RIVER I FRAME 80 76 DRIVER l 8-TABLE 84 CONTROLLER 2L I00 I56 VISUAL STEREO PCE PRESET DISPLAY AMP INDEXER I08 IL! SYNC v [60 W CIRCUIT T I32 AMP PREAMP 140 Y-DEFLN Y-SYNC CIRCUIT PATENTED APR 1 6 I974 SHEET 2 [IF 2 i Y-DEFLN CIRCUIT METHOD AND APPARATUS FOR OBTAINING A STEREOSCOPIC THERMAL IMAGE OF INTERNAL AND SURFACE PORTIONS OF AN ARTICLE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to methods and apparatus for exploring an article with infrared radiation, and in particular to methods and apparatus for obtaining a stereoscopic thermal image of constituent portions of the article by heating the article while simultaneously detecting the thermal responseof two discrete incremental areas scanned across a surface of the article.

2. Description of the Prior Art ln many manufacturing and testing operations, it is desirable to be able to determine certain internal and surface characteristics of an article without dismantling it, particularly if destruction of the article would result from the dismantling process. For example, if an electronic circuit, of a type having internal conductors which are not available for visual inspection, is tested and found to be defective as a result of an open or shorted conductor, it might be possible to repair the circuit if the exact location of the defect, or failure, is known. One technique for determining the internal and surface characteristics of an article is disclosed in copending U.S. Pat. application Ser. No. 373,348, filed on June 25, 1973, wherein a beam of radiant energy, directed onto an incremental area on a surface of the article, is scanned in a predetermined pattern across the surface while the thermal responses of at least two incremental areas on the surface, adjacent to the heated area, are detected. Electrical signals are generated in accordance with the detected thermal responses, and are employed to generate a visual thermal diffusogram of the surface Of the article. Since internal and surface mounted metal conductors, electronic hardware, insulating material, etc., of a circuit generally have differing thermal heating characteristics, heat will be carried to or away from the overlying surface area of the circuit in accordance therewith, and the thermal diffusogram of the circuit may be analyzed to determine the inner construction of the circuit and the location of defects. While this type of thermal diffusogram is useful in many cases for detecting defects within and on the surface of an article, it is a two dimensional representation of the interior and surface portions of an article, and is not useful in detecting many three dimensional type faults.

Two prior art techniques for three dimensional analysis of interior and surface portions of an article are x-ray laminography and stereoradiography. These techniques are not well adapted for precision work, however, because of difficulties in obtaining reasonable magnification and resolution when x-rays are directed and focused with lenses or other image forming optics. Furthermore, these techniques require expensive and complex equipment, and good x-ray shielding is necessary for their safe operation.

SUMMARY OF THE INVENTION ln accordance with the present invention, an apparatus for detecting a characteristic of an article includes optics for focusing a beam of radiation onto the article to heat the article, and two heat detectors, each for detecting, along an optical path, the thermal response of a separate one of two discrete incremental areas of the surface of the article and for generating a signal in accordance therewith. Further included is optics for scanning the optical paths of the detectors across the surface of the article, to scan successive incremental areas on the surface of the article, to generate signals from the heat detectors which vary in accordance with the thermal responses of the scanned incremental areas, and first and second visual display devices, each for receiving a signal from a separate one of the heat detectors, and for displaying an image representative of the thermal response detected by that heat detector as the optical path thereof is scanned across the surface of the article.

More particularly, the invention contemplates an apparatus for obtaining a three dimensional thermal image of constituent portions of an article both internal of the article and on the surface of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal pattern on a surface thereof which is a thermal image of internal and surface portions, and the optics for focusing the beam of radiation onto the article focus the beam of radiation onto an incremental area on a first surface of the article to heat the article. The two heat detectors each detect, along an optical path, the thermal response of a separate one of two discrete incremental areas on a second surface of the article which is opposite from and parallel to the first surface, with the detected incremental areas being in a spaced relationship with respect to each other and at a point on the second surface which is essentially opposite from the heated area on the first surface. Scanning the optical paths of the detectors across the surface of the article includes optics for cyclically and simultaneously scanning the optical paths across the second surface of the article in both first and second directions, which are orthogonal with respect to each other, and also included is optics for scanning the beam of radiation across the first surface simultaneously with, and in unison with, the scan of the optical paths of the detectors across the secondsurface.

The visual display devices are first and second cathode ray tubes, each for receiving the electrical signal from a separate one of the two heat detectors and for generating on a screen thereof a raster image which is scanned synchronously with, and in directions corresponding to, the scan of the optical paths of the detectors across the second surface of the article. The image on the screen of each tube varies in brightness in accordance with the received electrical signal, and defines a thermal image of the second surface of the article as detected by the heat detector which generated the electrical signal. With the spaced relationship between the detected incremental areas, the image on the screen of one cathode ray tube is a left hand thermal image of the heating experienced on the second surface of the article, while the image on the screen of the other cathode ray tube represents a right hand image of the thermal image on the second surface of the article, which images together define a stereoscopic visual image of the thermal image on the second surface of the article, and therefore a stereoscopic thermal image of the interior and surface portions of the article. Further includes is a mechanism, responsive after at least one scanning cycle of the optical paths of the detectors, for rotating the article about an axis through an incremental amount to cyclically change the parallax between the second surface of the article and the optical paths of the heat detectors.

Other objects, advantages and features of the invention will be apparent upon consideration of the following detailed description when taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows apparatus for generating a three dimensional thermal image of portions of an article, which are both interior of and on the surface of the article, in accordance with the teachings of the invention, and

FIG. 2 illustrates one type of visual stereoscopic display apparatus which may be employed with the apparatus of FIG. 1.

DETAILED DESCRIPTION Referring to FIG. 1, apparatus is shown for defining, in a three dimensional image, structural details which are both interior of, and on the surface of, an article, or workpiece 12. For the purpose of describing the invention, the article 12 shall be considered as having a planar configuration (an edgewise view of the article being illustrated in FIG. 1 it being understood that articles having other configurations may readily be used in the practice of the invention. A beam of radiant energy is focused to a heating spot 14 on an incremental area 16 on a first surface 20 of the article 12 to heat the area and, through the thermal conduction of the article, to heat the article. The beam, and therefore the heating spot 14, is then scanned, or'swept, across the surface 20 of the article 12 along a predetermined path, or in a predetermined pattern, to heat successive incremental areas 16 which lie along the path. The thermal response of the article 12, to being heated by the heating spot 14 at the incremental area 16, is detected at each of two exploring spots 22 which are images of each of two discrete and spaced incremental areas 24 on a second surface 28 of the article 12, opposite the first surface 20 and parallel thereto, focused with appropriate optics along an optical path and onto an associated separate one of two thermal detecting devices, such as the detectors 30.

The exploring spots 22 on the surface 28, or the images of the incremental areas 24 focused onto the thermal detecting devices, are at a position on the surface 28, with respect to the position of the heating spot 14 on the surface 20, which is essentially opposite the heating spot and slightly behind the heating spotwith respect to the direction of scan thereof. Optics, positioned in the optical paths of the images of the incremental areas 16 focused onto the thermal detectors, scan the thermal detectors across the surface 28 by scanning the optical paths thereof to scan the exploring spots 22 across the surface 28 in the predetermined pattern synchronously with, in unison with, and slightly behind the heating spot 14 with respect to the direction of scan thereof, and in a fixed spatial relationship with respect to each other. As the exploring spots 22 are scanned across the surface 28 in the predetermined path, successive incremental areas 24, which lie along the path, are focused onto the thermal detecting devices which detect variations in temperature, or radiation emission, at the incremental areas 24 in response to the article being simultaneously heated at the incremental areas 16 by the heating spot 14.

When the incremental areas 16 of the surface 20 are scanned by the heating spot 14, thermal energy is conducted into the article 12 and the temperature of the incremental areas 24 scanned by the exploring spots 22 is dependent upon the rate at which heat is conducted through the article 12. The magnitude of the tempera ture of the explored areas is largely dependent upon the structure, or type of material, out of which the article is constructed; however, variations in temperature will be effected if there are discontinuities in the structure of the material of the article 12, or if material of differing types, with differing thermal conductivities, is employed in the article. In the case of the article 12 being an electronic circuit, having internal conductors and hardware molded within and on the surface of insulating material, to be explored by the apparatus of the invention, the insulating portion of the circuit will absorb more heat from the heating spot 14 than will be conductors, and will therefore reach a higher temperature, and emit more radiation to be detected, than will a conductor. Conversely, a conductor will absorb less heat than an insulating portion of the circuit, and will therefore emit less radiation to be detected. More particularly, the radiation emitted by the surface 28 of the article 12, and detected with the exploring spots 22, is a function of the thermal characteristics of the constituent parts, or portions, of the article 12 immediately at and in close proximity with the exploring spots, and the thermal response exhibited on the surface 28 of the article, as detected by the scanned exploring spots 22, is a thermal image of the internal and surface portions of the article, or of the constituent parts of the article.

The thermal detecting devices, or radiation pickup devices, may be radiometers, and each generates an electrical signal in accordance with the intensity of the radiation detected thereby, or, in accordance with the temperature of as associated incremental area 24 of the surface 28, the image of which is focused thereon. Therefore, the electrical signal generated by each radiometer is a function of the temperature of the incremental area l6'being detected by the exploring spot 24.

To obtain an indication of the heating experienced on the surface 28 of the article 12 when the surface 20 of the article is scanned with the heating spot 14, and in the present invention to obtain a three dimensional thermal image of the constituent portions, or structural details, of the article 12 both interior of and on the surface of the article, the electrical signal generated by each thermal detector, in accordance with the temperature on the surface 28 as detected at its associated exploring spot 14, is applied, through appropriate circuitry, to a stereo display device 31 which, in the practice of the present invention, may be comprised of first and second cathode ray tubes (as shown in FIG. 2), each for receiving at a controlled grid thereof the signal from an associated separate one of the two thermal detectors. Each cathode ray tube generates on a screen thereof a point image, at the point where the electron beam from a gun in the tube strikes the screen of the tube, which image varies in brightness in accordance with the values of the electrical signal generated by the associated thermal detector. Therefore, the brightness of the point image on the screen of each cathode ray tube is in accordance, and representative of, the temperature, or radiation emission, as detected by a separate one of the exploring spots 22 on the surface 28 of the workpiece 12. A thermal diffusogram of the surface 28, which is a thermal image of the thermal response as detected by an exploring spot 22, is then obtained on the screen of each cathode ray tube for its associated exploring spot by scanning the electron beam thereof across the screen of the tube in the same pattern as, and in synchronism with, the scan of the exploring spots 22 across the surface 28. The thermal diffusogram on the screen of each cathode ray tube is, therefore, a thermal image representative of the internal and surface structure, or portions, of the article, as detected by a separate one of the exploring spots.

With the exploring spots 22 maintained in a spaced relationship with respect to each other, which spacing corresponds to an appropriate base line for stereoscopy (such as 65 mm base of normal interocular distance to correspond to orthostereoscopic presentation), a left hand thermal image of the structure of the article 12 is displayed on the screen of one of the cathode ray tubes, and a right hand thennal image of the structure of the article 12 is displayed on the screen of the other cathode ray tube, which images together form a stereoscopic, or three dimensional, thermal image of the constituent portions of the article. By cyclically rotating the article 12 an incremental amount about an axis to cyclically change the parallax, or angle, between the optical paths of the thermal detectors and the surface 28 of the article 12, or between the surface 28 and the incremental areas 24 as detected by the thermal detectors, the stereoscopic effect of the thermal image may be increased. In this manner, if an operator testing the article 12 observes the image on each screen with a separate one of his eyes, he will perceive a stereoscopic representation of the thermal image exhibited by the structure of the article 12 both interior of, and on the surface of, the article, irrespective of whether or not the material of the article 12, surrounding the structural portions thereof, would otherwise be opaque to his unaided eyes.

More particularly, in practicing the present invention the radiation output from a source of infrared (IR) radiation 32 is controlled in intensity by a conventional,

variable power supply 36, and is focused into a point source with a collector lens 40 after passing through a narrow bandpass filter 44. The bandpass range of the filter 44 is chosen such that the radiation passing therethrough is of a wavelength to which the particular material of the article 12, which encapsulates the structural portions thereof, exhibits a high transmittance. In this manner, a greater proportion of the total energy of the IR beam striking the surface of the article 12 at the heating spot 14 is passed through the encapsulating material thereof, for impingement on the inner structural portions of the article, than would otherwise occur if a broad-band IR beam were employed to irradiate the article.

The collector lens 40' is essentially a bullet-shaped lens with a flat base and a narrow, pointed tip, and in the instant case is made of high refracting IR transmitting materials such as sapphire, quartz or barium fluolens and focuses the radiation into a narrow beam. The focused beam of radiation passing from the focusing optics 48 is directed onto a line scanning mirror 52, oscillatingly driven by a line driver oscillator 56 through an electromechanical transducer, such as a piezoelectric crystal, which oscillates, in response to a controlled input to the oscillator 56, 12,000 times per second in an x-direction to scan, or sweep, the beam of radiation twelve thousand lines per second in the xdirection. The line scanning plane mirror 52, as well as the piezoelectric device and the line driver oscillator 56, may be an integral optical scanning unit, such as a Model No. G 0606 unit sold by General Scanning, Inc. of New York, New York. The line scanning plane mirror 52 reflects the beam of radiation onto a frame scanning plane mirror 60, which in turn reflects the beam of radiation onto the surface 20 of the article 12 as the heating spot 14. The plane scanning mirror 60 is oscillatingly driven by a frame driver oscillator 64 through an electromechanical transducer, such as a piezoelectric crystal, and oscillates, in response to a controlled input to the oscillator 64, 60 times per second in a y-direction, which is orthogonal with respect to the x-direction, to scan, or sweep, the beam of radiation 60 frames per second in the y-direction. The frame scanning plane mirror 60, as well as the piezoelectric device and the frame driver oscillator 64, maybe an integral optical scanning unit, such as a Model No. 0-330 unit also sold by General Scanning, Inc. It is seen, therefore, that the mirrors 52 and 60, which together direct the beam of radiation onto the surface 20 as the heating spot 14, together operate to scan the beam of radiation, and therefore the heating spot 14, in an x and a y direction, or in lines and frames, across the surface 20.

To detect the thermal response of the article 12 to being heated by the beam of radiation as the heating spot 14 is scanned across the surface 20, the image of the incremental area 24 at each exploring spot 22 on the surface 28, and therefore the radiation emitted from the incremental area at the exploring spot, is transmitted along an optical path to a separate and associated one of two thermal detecting devices, or radiometers, 68 or 72, which are the detectors 30, through the use of additional optics. Specifically, a second frame scanning plane mirror 76, which is similar to the frame scanning plane mirror 60, directs the image at each of the exploring spots 22 on the surface 28 onto a line scanning plane mirror 80. The frame scanning plane mirror 76 is oscillatingly driven by the frame driver oscillator 64 through an electromechanical transducer, such as a piezoelectric crystal, and oscillates, in response to the output from the oscillator 64, 60 times per second in the y-direction to scan, or

sweep, 60 frames per second in the y-direction with the exploring spots 22 on the surface 28 of the workpiece 12. The frame scanning mirror 76, as well as the piezoelectric device and the frame driver oscillator 64, similar with the frame scanning mirror 60, its piezoelectric device and the frame driver oscillator 64, may also be an integral optical scanning unit, such as the Model No. G-33O unit sold be General Scanning, Inc. In the instant invention, although a frame driver oscillator 64 is included in both of the optical scanning units, for driving the frame scanning plane mirrors 60 and 72, only the frame driver oscillator from one of the scanning units is employed to drive each of the frame scanning plane mirrors 60 and 72, through their associated piezoelectric crystals, to maintain sweeping synchronism and a predetermined spatial relationship between the heating spot 14 and the exploring spots 22 as they are scanned in the y-direction.

The line scanning plane mirror 80 is oscillatingly driven by the line driver oscillator 56 through an electromechanical transducer, such as a piezoelectric crystal, and oscillates, in response to the controlled input to the oscillator, 12,000 times per second in the xdirection to scan, or sweep, 12,000 lines per second in the x-direction with the exploring spots 22 on the surface 28 of the workpiece 12. The line scanning plane mirror 80, as well as the piezoelectric device and line driver oscillator 56, similar with the line scanning plane mirror 52, its piezoelectric device and the line driver oscillator 56, may also be an integral optical scanning unit, such as the Model 6-0606 unit sold by General Scanning, Inc. In the present invention, while each of the line scanning plane mirrors 52 and 80 are associated with an individual integral optical scanning unit, only the line driver oscillator 56 from one of the scanning units is employed to oscillate the mirrors 52 and 80, through their associated piezoelectric crystals, to maintain synchronism and a predetermined spatial relationship between the heating spot 14 and the exploring spots 22 as they are scanned across their associated surfaces in the x-direction. It is seen, therefore, that the use of the common line driver oscillator 56 to oscillate the line scanning planemirrors 52 and 80, and the use of the common frame driver oscillator 64 to oscillate the frame scanning plane mirrors 60 and 76, enables the heating spot 14 and the exploring spots 22 to be scanned in unison, and in a predetermined spatial relationship with each other, across their associated sur-.

faces of the workpiece 12 I in the x and in the y directions. i

The images of the incremental areas 24 at the exploring spots 22 are each reflected from the line scanning plane mirror 80 onto a stationary plane mirror 84. The plane mirror 84 in turn reflects each image onto a separate one of the two thermal detecting devices 68 and 72 through the use of detection focusing optics 88, such as two germanium planoconvex lenses, and a wedge shaped reflector 92 which reflects a separate one of images from each of its sides onto an associated stationary plane mirror 96 or 100 for reflection therefrom onto one of the detectors 68 or 72. Each lR detector 68 and 72 is positioned at the image plane of the detection focusing optics 88, and is positioned with respect to the other such that the image of the incremental area 24 detected thereby is spaced, with respect to the image of the other incremental area, at a stereoscopic parallax which corresponds to an appropriate base line for later stereoscopic viewing by an operator (such as 65 mm base of normal interocular distance to correspond to orthostereoscopic presentation), and such that the images lie along a line parallel to the x-direction on the surface of the article. The thermal detectors 68 and 72 may be Model No. DLK-39 mercury cadmium telluride infrared detectors sold by Honeywell Corp., of Minneapolis, Minnesota, and are cooled by a source of liquid nitrogen (not shown) for proper operation.

Each thermal detector 68 and 72 generates an electrical signal having a value in accordance with, or which corresponds to, the temperature of, of the intensity of the radiation emitted from, an incremental area 24 on the surface 28, the image of which is focused thereon. Therefore, as the exploring spots 22 are scanned across the surface 28, the electrical signals from the detectors 68 and 72, as successive images of incremental areas 24 are focused thereon, are a function of the heating experienced by the surface 20 along the path of the exploring spots, Since the heating experienced by the surface 28 upon heating the article 12 with the heating spot 14 is a function of the thermal characteristics of the structure, or constituent portions, of the article, both interior of the article and on the surface of the article, and is therefore a thermal image of the structure of the article, the signals generated by the detectors 68 and 72 are each representative of the thermal image of the structure of the article 12 as detected at a separate one of the exploring spots 22, as the exploring spots are scanned across the surface 28.

To obtain a visual representation of the heating experienced by the surface 28 of the workpiece 12 upon the heating of the surface 20 with the heating spot 14, or to obtain a thermal image of the structure of the workpiece 12 as exhibited on the surface 28, the electrical signal from each thermal detector 68 and 72 is applied through a preamplifier circuit 104and an amplifier circuit 108 to a separate one of two inputs of the visual stereo display device 31. The visual stereo display device 31 may be, by way of example and as shown in FIG. 2 of the drawings, first and second cathode ray tubes 116 and 120, each of which'receives a separate one of the outputs from the video amplifier 108 at a control grid thereof for controlling the brightness of an electron beam on a screen of the tube in accordance with the value of the signal applied to the control grid, or in accordance with'the intensity of the radiation detected at one of the exploring spots 22 by an associated one of the radiation detectors 68 or 72, In other words, the electron beam of each cathode ray tube 116 and 120 is controlled in intensity by the radiation emission of the surface 28 of the article 12 as detected by a separate one of the exploring spots 22.

To obtain a raster image, on the screen of each cathode ray tube 116'and 120, representative of the thermal image on the surface 28 as detected by a separate one of the scanned exploring spots 22, an x-deflection circuit 124 drives deflection coils 128 associated with each of the tubes 116 and 120 to scan, or sweep, raster lines on the screen thereof in an x-direction at a rate of 12,000 lines per second, and a y-direction circuit 132 drives deflection coils 136 associated with each of the tubes 116 and 120 to scan raster frames on the screens thereof in a y-direction at a rate of 60 frames per second. The x-direction circuit 124 also provides the controlled input to the line driver circuit 56 to oscillate the line scanning plane mirrors 52 and in the x-direction at the rate of 12,000 oscillations per second, and the y-direction circuit 132 similarly provides the controlled input to the frame driver circuit 64 to oscillate the frame scanning plane mirrors 60 and 76 in the ydirection at the rate of 60 oscillations per second. The x and y directions of sweep on the screens of the cathode ray tubes 116 and correspond to the x and y directions scanned by the exploring spots 22 on the surface 28 of the article 12 and are, therefore, orthogonal with respect to each other. i

It is seen, therefore, that the heating spot 14, and the exploring spots 22, scan lines and frames on their associated surfaces of the article 12 at the same rate, and

in directions corresponding with, the sweep of the electron beams in lines and frames on the screens of the cathode ray tubes 116 and 120. Since the point image on the screen of each tube, formed by the electron beam of the tube impinging thereon, varies in brightness in accordance with the temperature of an incremental area 24 on the surface 28 as focused onto a separate one of the radiometers 68 or 72, it is only necessary to synchronize the initiation of the scans, and the location at which the scan begins, of both the exploring spots 22 on the surface 28 of the article 12 and of the electron beams on the screens of the cathode ray tubes 116 and 120, to obtain on the screen of each cathode ray tube a raster image which is a thermal image of the heating experienced on the surface 28 of the article 12 at successive incremental areas 24 along the path scanned by a separate one of the exploring spots 22. This synchronization is accomplished with a synchronization circuit 140, which is a common cathode relaxation oscillator, and which cyclically provides an input to both the x-direction circuit 124 and to the ydirection circuit 132 to momentarily disable, or blank out, the circuits, and to then restart the circuits at the same time. During the time that the circuits 124 and 132 are blanked out, the electron beams of the tubes 116 and 120 are returned to a predetermined position on the screens of the tubes, and the line driver circuit 56 and the'frame driver circuit 64 direct the exploring spots 22 and the heating spot 14 to a predetermined position on their associated surfaces of the article 12, so that when the circuits 124 and 132 are restarted the electron beams of the tubes 116 and 120, and the exploring spots 22 on the surface 28 will be in registry througout their scan of lines and frames through the predetermined pattern.

With the exploring spots 22 spaced a distance apart to obtain a desired stereoscopic parallax, the image on the screen of one of the cathode ray tubes, such as the tube 116, is a left hand image of the thermal image as detected on the surface 28 by a separate one of the exploring spots 22, and the image on the screen of the other cathode ray tube 120 is a right hand image of the thermal image as detected on the surface 28 by the other exploring spot 22, which images, when viewed one with each eye of an observer, define for the observer a stereoscopic thermal image on the surface 28 of the article 12. Such viewing by an observer may be accomplished, with the screens of the cathode ray tubes 1 16 and 120 in an opposing relationship as shown in FIG. 2, by positioning a wedge shaped reflector 144 between the screens for reflecting the image on the screen of the tube 116 to a left eyepiece 148 for viewing by the left eye of an observer, and for reflecting the image on the screen of the tube 120 to a right eyepiece 152 for viewing by the right eye of 'an observer. In this manner, the thermal images of the articles on the screens of the cathode ray tubes, each of which is a thermal image of structure interior of the article and on the surface of the article as detected by a separate one of the exploring spots 22, and the structure of the article having points spaced in a three dimensional relationship with respect to each other, such as points A, B and C, will, when simultaneously viewed one with each eye of an observer at the eyepieces 148 and 152,

define for the observers perception a three dimensional thermal image of the structure of the article having the corresponding points a, b and c spaced in their three dimensional relationship. It is understood, of course, that a stereoscopic presentation of the thermal image of the article may be obtained with a visual display device other than that described, such as by an anaglyphic method to allow simultaneous observation by a number of viewers, or through the use of a single cathode ray tube, the raster on the screen of which defines two separate images.

A maximum amount of thermal data for an effective stereoscopic presentation of the thermal image of the constituent portions of the article 12 is obtained by cyclically rotating the article an incremental amount about an axis parallel to the y-direction, as it is scanned by the heating spot 14 and by the exploring spots 22, to cyclically change the parallax between the optical paths of the detectors 68 and 72 and the surface 28 of the article 12, or between the exploring spots 22 as detected by the detectors and the surface of the article. In the preferred practice of the invention, the article is rotated in one .direction of rotation through 5 increments, after each scan of one or more frames across the surface 28 by the exploring spots 22, until the article has been rotated 45, and is then rotated through 5 increments in the opposite direction of rotation until it has again been rotated through 45. This rotation of the article continues throughout the time that the article is scanned by the exploring'spots 22, and is centered about an article position at which the optical paths of the detectors are perpendicular to the surface 28. In this manner, with the spaced exploring spots 22 lying along a line parallel to the x-direction, the thermal image of the structure of the article 12 on the screen of each cathode ray tube is cyclically and rapidly presented as viewed from separate and discrete angles. With a medium persistence screen on each cathode ray tube, this displays for an operators viewing a stereoscopic thermal image of the structure of the article which clearly defines not only the side portions of the structure, but also those structural portions of the article which underlie other structure.

Rotation of the article afer each occurrence of a predetermined number of frames scanned in the ydirection across the surface 28 by the exploring spots 22 is accomplished by applying the output from the synchronization circuit through a preamplifier circuit 156 to the input of a preset indexer 160. In the present invention, the output from the synchronization circuit 140 is a 0.2 millisecond pulse occurring every one sixtieth of a second for resetting, and for blanking or inhibiting during its duration, the x-deflection circuit '124 and the y-deflection circuit 132. The preset indexer 160 is presettable'to generate an output therefrom after each input thereto of a predetermined number of pulses from the synchronization circuit 140, which number of pulses are representative of a like number of frames'scanned by the exploring spots 22 on the surface 28 of the article 12. The output from the preset indexer 160 may be of either a first or a second type, and the preset indexer is also presettable to alternately provide a preset number of outputs of the first type and the same preset number of outputs of the second type.

The output from the preset indexer 160 occurs during the duration of an output pulse from the synchronization circuit 140, during which time the x-deflection circuit 124 and the y-deflection circuit 132 are inhibited, and is supplied to a 6 table controller, or motor 164, for controlling the rotation of a 6 table 168 upon which the article 12 is supported for rotation about an axis parallel to the y-direction. The arrangement is such that an output of a first type from the preset indexer 160 rotates the table 168 through a rotation in the opposite direction of rotation. The preset indexer 160 may, therefore, be preset to rotate the article 12 in one direction of rotation through 5 increments after a predetermined number of frames have been scanned by the exploring spots 22 on the surface 28 (usually one or two frames in the preferred practice of the invention), until the article has been rotated through 45, and to then rotate the article through 5 increments in the opposite direction of rotation until the article has again been rotated through 45 to cyclically change the parallex between the optical paths of the detectors 68 and 72 and the surface 28 of the article. In the present invention the preset indexer 160 is a Model 2SPl800-4 preset indexer, and the 0 table controller 164 is a Model HSSOD stepping motor, both of which are sold by the Superior Electric Company of Bristol, Connecticut. The operating rate of the preset indexer 160 and 0 table controller 164 is sufficiently fast so that each 5 rotation of the 0 table 168 occurs during the 0.2 millisecond duration of the output pulse from the synchronization circuit 140, during which time the x and y deflection circuits, and therefore the images on the screens of the cathode ray tubes 116 and 120, are blanked out, and the duration for which the article is explored at a particular parallax (two-sixtieths of a second for two frames) is sufficiently brief that, with a medium persistence screen on each cathode ray tube, and with a line scan rate of 200 lines per frame, the thermal images thereon appear to be continuous, complete, and without flicker.

In a contemplated use of the apparatus of the invention, the article 12 is a multilayer circuit board or module having a plurality of conductors and electric components encapsulated within, and carried on the surface of, an epoxy glass substrate which is opaque to the unaided eye. In this case, where it is desirable to detect the thermal responses of the conductors and electric components, or of the structural portions of the board, to IR radiation, and not the thermal response of the epoxy glass substrate, to obtain a stereoscopic thermal image thereof, the narrow bandpass filter 44 is chosen to pass therethrough only those wavelengths of the IR radiation to which the epoxy glass substrate exhibits a high transmittance. In this manner, a greater proportion of the energy in the beam of radiation, as directed onto the surface 20 of the article 12 at the heating spot 14, is passed through the epoxy glass substrate and absorbed by the conductors and components within the substrate than would be absorbed if broad band radiation were directed onto the heating spot 14. Accordingly, the thermal response on the surface 28 of the article 12 is more clearly representative of the thermal image of only the conductors and components of the multi-layer circuit board. Therefore, as the energy of the heating spot 14 is absorbed more by the conductive portions of the article, than by the epoxy glass insulating portion which exhibits a high transmittance to the wavelength of the energy, the conductor portions will be heated more than the insulating portion and will emit broad band radiation. This radiation is detected by t the thermal detectors 68 and 72, at the exploring spots 22, which generate electrical signals to provide on the screens of the cathode ray tubes 116 and left and right hand thermal images of the conductive and component portions of the circuit, which together define a stereoscopic thermal image of the conductive and component portions, or structural portions, of the circuit. If desired, the sensitivity of the detectors 68 and 72 to the radiation emitted by the conductive portions of the circuit may be increased by positioning a spectral filter, which filters out the particular spectra passed through the filter 44, in front of the detectors.

In the foregoing description of the invention, heating of the article 12 was accomplished by directing a beam of radiation to a heating spot 14 on an incremental area 16 of the surface 20, essentially opposite from the exploring spots 22 on the surface 28, and by then scanning the heating spot 14 and the exploring spots 22 across their associated surfaces in synchronism with, and in unison with, each other. In this manner, the article may be thermally explored with a minimum amount of energy, or heat, input thereto. It should be appreciated, however, that many other techniques may be employed to heat the article. For example, the entire surface 28 of the article 12 may be irradiated with a beam of radiation to heat the article, or the article may simply be heated in an oven, the particular technique employed in heating the article being a matter of choice to one skilled in the art.

While one particular embodiment of the invention has been described in detail, it is understood that various other modifications and embodiments may be devised by one skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In a system for generating a stereoscopic thermal image of constituent parts of an article:

means for heating the article;

means, responsive to the heating experienced by the constituent parts of the article, as detected at pairs of discrete areas on a surface of the article, for generating a pair of signals which vary in accordance with the heating experienced by the constituent parts, and

means, responsive to the pair of signals, for producing a stereoscopic thermal image of the constituent parts of the article.

2. In a system for generating a stereoscopic thermal image of constituent parts of an article;

means for scanning a beam of heat energy across a first surface of the article to heat the article;

means, responsive to heat energy radiated by the heated constituent parts, at pairs of spaced discrete areas on a second surface of the article which is opposite from the first surface of the article, for generating a pair of signals which vary in accordance with the heat energy radiated by the constitutent parts as the beam of heat energy scans the first surface, and

means, responsive to the pair of signals, for producing a stereoscopic thermal image of the constituent parts of the articles.

3. In a non-contacting apparatus for generating a stereoscopic thermal image of constituent parts of an article:

means for transferring heat to the article;

two heat detector means, each for detecting the thermal response of the constituent part of the article at a separate one of two discrete incremental areas the heat detector means and for producing a stereoscopic thermal image of the constituent parts of the article.

4. In the noncontacting apparatus as set forth in claim 3, wherein:

the means for transferring heat to the article includes means for directing infrared radiation onto a first surface of the article;

the two heat detector means include two radiation pickup devices which are each responsive to detected infrared radiation from a separate one of two incremental areas on a second surface of the article to generate a signal which is representative of the radiation emitted by the article at the detected incremental areas on the second surface as the article is heated by the radiation on the first surface, and

the means for scanning the heat detector means over the surface of the article includes optical scan means for cyclically scanning each of the pickup devices over the second surface in a predetermined 'pattem and in unison with each other.

5.ln a noncontacting apparatus as set forth in claim 5, wherein the means for cyclically changing the parallax includes:

means, cyclicallyresponsive after at least one cycle of the optical scan means, for rotating the article about an axis through an incremental amount until the article has been rotated through a predetermined number of incremental amounts in one direction of rotation, and for then rotating the article through thepredetermined number of incremental amounts inth'eopposite direction of rotation, to cyclically change theparallax between the second surface of the article and the scanned incremental areas thereon as detected by the pickup devices.

6. In a noncontacting apparatus for detecting a characteristic of an article:

means for transferring heat to an incremental area of a first surface of the article to heat the article;

two heat detectors, each for detecting the thermal response of a separate one of two discrete incremental areas of a second surface of the article and for generating a signal in accordance therewith;

means for cyclically scanning the heat transfer means and the heat detectors across their associated article surfaces in a predetermined pattern to heat successive scanned incremental areas on the first surface and to generate signals from the heat detectors which vary in accordance with the thermal responses of successive scanned incremental areas on the second surface;

means, cyclically responsive after at least one cycle of the scanning means, for rotating the article about an axis through an incremental amount until the article has been rotated through a predeter- LII mined number of incremental amounts in one direction of rotation, and for then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclically change the parallax between the second surface of the article and the scanned incremental areas thereon as detected by the heat detectors, and means for indicating the value of the signals generated by the heat detectors as the heat detectors are scanned across the second surface. 7. In a noncontacting apparatus as set forth in claim 6, wherein:

the means for transferring heat to an incremental area of the first surface to heat the article includes means for directing a beam of infrared radiation onto the area; the two heat detectors include two radiation pickup devices which are each focused along an optical path onto a separate one of the two incremental areas and which are each responsive to the detected infrared radiation from the detected incremental area on the second surface to generate a signal in accordance therewith, and the means for scanning the heat transfer means and the heat detectors across their associated surfaces includes optical scan means for cyclically scanning the beam of infrared radiation and the optical paths of the radiation pickup devices across their associated surfaces in the predetermined pattern. 8. In the noncontacting apparatus as set forth in claim 7, wherein the means for indicating the value of the signals generated by the heat detectors includes:

amplifier means, for receiving the signals from the pickup devices, and for generating output signals in accordance therewith, and

visual indicator means, responsive to the signals from the amplifier means to generate two separate indications, each of which is representative of the radiation detected by a separate one of the pickup devices as the optical path thereof is scanned across the second surface of the article.

9. In a scanning apparatus for inspecting a workpiece which exhibits a varyingthermal heating patters in response to heating thereof:

means for focusing a beam of radiation onto an incremental area of a first surface of the workpiece to heat the workpiece;

two radiation sensing devices for receiving radiation and for generating an electrical signal in accordance with the intensity of the received radiation;

means for focusing each radiation sensing device along an optical path and onto a separate one of two discrete incremental areas of a second surface of the workpiece, the second surface being opposite from and parallel to the first surface, and the incremental areas on the second surface being essentially opposite from the heated incremental area on the first surface, to generate an electrical signal with each sensing device in accordance with the radiation emitted by the incremental area on the second surface onto which the sensing device is focused, the radiation being in accordance with the thermal heating pattern exhibited by the article at that incremental area upon the heating of the article by the beam of radiation;

means for cyclically scanning the beam of radiation and the optical path of each radiation sensing device across their associated workpiece surfaces in unison with each other to generate electrical signals with the sensing devices which vary in accordance with the radiation emitted by the successive scanned incremental areas on the second surface of the article;

means, cyclically responsive after a predetermined number of cycles of the scanning means, for rotating the workpiece an incremental amount about an axis, to cyclically change the parallax between the thermal pattern exhibited by the workpiece on the second surface and the optical paths of the radiation sensing devices, and

means for indicating the value of the electrical signals generated by the sensing devices as the optical paths of the sensing devices are scanned across the second surface of the workpiece.

10. In a scanning apparatus as set forth in claim 9,

wherein:

each radiation sensing device is a radiometer; the scanning means includes a. first and second plane mirrors, each adapted for rotational oscillation about an axis, the first plane mirror being positioned in the path of the beam of radiation, and the second plane mirror being positioned in the optical paths of the radiometers, to scan the beam of radiation and the optical paths of the radiometers across their associated surfaces in a first direction;

b. means for cyclically oscillating the first and the second plane mirrors about their axes;

c. third and fourth plane mirrors, each adapted for rotational oscillation about an axis, the third plane mirror being positioned in the path of the beam of radiation, and the fourth plane mirror being positioned in the optical paths of the radiometers, to scan the beam of radiation and the optical paths of the radiometers across their asso-' ciated surfaces in a second direction which is orthogonal with respect to the first direction, and

d. means for cyclically oscillating the third and the fourth plane mirros about their axes, and the means for indicating the value of the electrical signals generated by the radiometers includes a. first and second cathode ray tubes, each having an electron gun which receives an input which varies in accordance with the electrical signal from a separate one of the radiometers and which generates on a screen of the tube a raster which correspondingly varies in accordance with the electrical signal;

b. first sweep means, synchronously coupled with the means for oscillating the first and second plane mirrors about their axes, for sweeping the raster across the screen of each cathode ray tube in a direction which corresponds with the first direction, and

0. second sweep means, synchronously coupled with the means for oscillating the third and fourth plane mirrors about their axes, for sweeping the raster across the screen of each cathode ray tube in a direction which corresponds with the second direction to generate, in cooperation with the first sweep means, an image on the screen of each tube which is representation of the thermal heating pattern on the second surface of the workpiece as detected by a separate one of the radiometers as the optical path thereof is scanned across the second surface of the workpiece.

11. In an apparatus for obtaining a three dimensional thermal image of constituent portions of an article both internal of the article and on the surfaces of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal response on a surface thereof which is a thermal image of its constituent portion:

means for heating the article; path to generate from the thermal response detecting means signals means for detecting the thermal response of each of two spaced incremental areas on a surface of the article and for generating a separate signal in accordance with each detected thermal response;

means for cyclically scanning the thermal response detecting means across the surface of the article in a predetermined means signals which vary in accordance with the thermal responses exhibited by the successive scanned incremental areas on the surface which lie along the predetermined path;

means, cyclically responsive after at least one cycle of the scanning means, for rotating the article an incremental amount about an axis to vary the parallax between the surface of the article and the incremental areas as detected by the detecting means, and

a visual display device, having a viewing screen, for receiving the separatesignals from the detecting means and for generating on the screen thereof a separate visible image, in accordance with each signal, which is a visual representation of the thermal response exhibited on the surface of the article along the predetermined path as the thermal detecting means is scannedtherealong, the separate visible images, with the spaced relationship of the two detected incremental areas, and with the cyclically varied parallax between the surface of the article and the incremental areas as detected by the detecting means, being left and right hand images of the thermal heating pattern exhibited by the article along'the predetermined path, and together being a stereoscopic representation of the thermal image of the constituent portion of the article.

12. In an apparatus for obtaining a three dimensional thermal image of constituent portions of an article both internal of the article and on the surfaces of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal pattern on a surface thereof which is a thermal image of its constituent internal and surface portions:

means for focusing a beam of radiation onto an incremental area of a first surface of the article to heat the area and, by the thermal conduction of the article, to heat the article;

two radiation pickup devices, each for receiving radiation and for generating a signal in accordance with the intensity thereof;

means for focusing each radiation pickup device along an optical path and onto a separate one of two spaced incremental areas on a second surface of the article, the second surface being opposite from and parallel to the first surface, and the incremental areas on the second surface being essentially opposite from the heated-incremental area on the first surface, to generate a signal with each radione of the video display devices displays on its screen a left hand image of the thermal heating pattern on the second surface of the article, while the other video display device displays on its screen a ation pickup device in accordance with the radiaright hand image of the thermal heating pattern on tion emitted by the incremental area on the second the second surface of the article, which, taken tosurface onto which the pickup device is focused, gether, are a stereoscopic representation of the the radiation being in accordance with the thermal thermal image of the constituent internal and surheating pattern exhibited by the article at that inface portions of the article. cremental area upon the heating of the article by 13. In an apparatus as set forth in claim 12, further the beam of radiation; including:

first scanning means for cyclically scanning the beam means for filtering the beam of radiation to pass, to of radiation and the optical paths of the radiation the first surface of the article, a beam of radiation pickup devices across their associated surfaces in which is transmissible with respect to one of the a first direction and in unison with each other; constituent portions of the article.

14. In an apparatus as set forth in claim 12, wherein the first and second directions are orthogonal with respect to each other and the detected incremental areas lie along a line parallel to the first direction, and wherein:

second scanning means, simultaneously operative with the first scanning means, for cyclically scanning the beam of radiation and the optical paths of the radiation pickup devices across their associated surfaces in a second direction, to generate with the pickup devices signals which vary in accordance with the radiation emitted by the successive detected incremental areas on the second surface of the article as the article is simultaneously heated by the beam of radiation on the first surface;

first and second video display devices, each having a screen for displaying video information thereon, and each for receiving at an input thereof a signal from a separate one of the radiation pickup devices and for generating on its screen a point image which varies in brightness in accordance with the value of the signal at its input, and therefore in accordance with the intensity of the radiation detected by the pickup device;

paths of the radiation pickup devices and the second surface of the article, so that, with the spaced relationship of the two detected incremental areas,

the radiation pickup devices are radiometers;

the first and second video display devices are first and second cathode ray tubes, each having an electron gun for being controlled by the signal from a separate one of the radiometers and for generating, with an electron beam therefrom, the variable brightness point image on a screen of the tube;

the first sweep means is a first deflection circuit for sweeping the electron beam from the gun in each cathode ray tube across the screen of the tube in a direction corresponding with the first direction;

the second sweep means is a second deflection circuit for sweeping the electron beam from the gun in each cathode ray tube across the screen of the tube first sweep means, synchronouslyv coupled with the in a direction corresponding with the second direcfirst scanning means, for cyclically sweeping the tion; point image on each screen across the screen in a the first scanning means includes direction corresponding to the first direction, and a7 first and second plane mirrors adapted for oscilin synchronism with the scan of the optical paths of lation about axes in synchronism with each other, the pickup devices across the second surface of the 40 the first plane mirror being in the path of the article in the first direction; beam of radiation and the second plane mirror second sweep means, synchronously coupled with being in the optical'paths of the radiometers, for the second scanning means, for cyclically sweeping scanning the beam of radiation and the optical the point image on each screen across the screen paths of the radiometers across their associated in a direction corresponding to the second direcarticle surfaces in the first direction, and tion, and in synchronism with the scan of the optib. first and second piezoelectric devices coupled to cal paths of the pickup devices across the second the first and second plane mirrors, respectively, surface of the article in the second direction, to disfor responding to a signal from the first deflecplay on each screen, as the brightness of the point tion circuit to impart oscillating movement to the image thereon varies in accordance with the intenfirst and second plane mirrors in accordance sity of the radiation detected by a separate one of therewith to scan the beam of radiation across the radiation pickup devices, a visual representathe first surface of the article, and the optical tion of the thermal heating pattern on the second paths of the radiometers across the second sursurface of the article as detected by a separate one face of the article in the first direction in synof the pickup devices; chronism with the sweep of the electron beams means, cyclically responsive after at least one cycle across the screens of the cathode ray tubes in the of the second sweep means, forrotating the article direction corresponding with the first direction; about an axis through an incremental amount in one direction of rotation until the article has been the second sweep means includes rotated through a predetermined number of increa. third and fourth plane mirrors adapted for oscilmental amounts, and for then rotating the article lation about axes in synchronism with each other, through the predetermined number of incremental the third plane mirror being in the path of the amounts in the opposite direction of rotation, to beam of radiation and the fourth plane mirror cyclically change the parallax between the optical being in the optical paths of the radiometers, to

scan the beam of radiation and the optical paths of the radiometers across their associated surfaces in the second direction, and

b. third and fourth piezoelectric devices coupled to the third and fourth plane mirrors, respectively, for responding to a signal from the second deflection circuit to impart oscillating movement to the third and fourth plane mirrors in accordance therewith, to scan the beam of radiation across the first surface of the article and the optical paths of the radiometers across the second surface of the article in the second direction in synchronism with the sweep of the cathode ray tubes in the direction corresponding with the second direction, and

the means for rotating the article about an axis through incremental amounts is a numberic controlled 6 table coupled to the second deflection circuit, for rotating the article about an axis parallel to the second direction.

15. In a method of producing a stereoscopic image indicative of thermal characteristics of constituent parts of an article:

heating the article to produce a thermal response gradient across a surface of the article which varies in accordance with the thermal characteristics of the constituent parts;

generating a pair of signals which vary in accordance with the thermal response gradient at a succession of pairs of discrete areas across the surface of the article, and

producing, in accordance with the pair of signals, a

stereoscopic image indicative of the thermal characteristics of the constituent parts of the article.

16. In a method of generating a stereoscopic thermal image of the constituent parts of an article:

heating the article;

sequentially detecting the thermal response of the constituent parts of the article at each of a first plurality of successive incremental areas which lie along a predetermined path on a surface of successive incremental areas which lie along the predetermined path article;

generating a first electrical signal which varies in accordancewith the detected thermal response of the first plurality of successively detected incremental areas;

sequentially detecting, simultaneously with detecting the thermal responses of the first plurality of incremental areas, the thermal response of the constituent parts of an article at 'each of a second plurality predetermined path on the surface of the article, the detected incremental area of the first plurality of incremental areas and the simultaneously'detected incremental area of the second plurality of incremental areas being at a fixed spacing with respect to each other;

generating a second electrical signal which varies in accordance with the detected thermal response of the second plurality of successively detected incremental areas;

generating a first visual image, in accordance with the first electrical signal, representative of the thermal response detected at each of the first plurality of incremental areas, and

generating a second visual image, in accordance with the second electrical signal, representative of the thermal response detected at each of the second plurality of incremental areas the first and second images beng left and right hand thermal images respectively which, taken together, form a stereoscopic thermal image of the constituent parts of the article. 17. In a method of generating a stereoscopic thermal image of constituent parts of an article:

heating the article; focusing each of two radiation pickup devices along an optical path at an angle with respect to a surface of the article, and onto a separate one of two spaced and discrete incremental areas on the surface to detect the thermal radiation emitted by the constituent parts thereat; generating a separate electrical signal in accordance with the radiation detected by each radiation pickup device; scanning the optical paths of the radiation pickup devices along a path on the surface of the article to detect with each pickup device the radiation emitted by successive scanned incremental areas to generate signals which vary in accordance with the radiation emission of the scanned incremental areas; cyclically changing the angle between the optical paths of the radiation pickup devices and the surface of the article, and generating in accordance with the value of the electrical signals, a stereoscopic thermal image of the constituent parts of the article. 18. In a method as in claim 17, wherein: the scanning step includes cyclically scanning the optical' paths of the pickup devices in unison with each other across the second surface of the article in a predetermined path to maintain the detected scanned incremental areas in a fixed spaced relationship with respect to each other; the indicating step includes generating first and second images, each of which is in accordance with the value of a separate one of the two electrical signals and is representative of the radiation emitted by the surface of the article along the predetermined path as detected by a separate one of the two pickup devices, and cyclically changing the angle between the optical paths of the pickup devices and the surface of the article includes cyclically rotating the article an incremental amount about an'axis, in response to at least one scan of the optical paths of the pickup devices across the surface of the article, so that with the spaced relationship of the detected incremental areas the first image is a left hand image of the radiation emitted by the surface of the article along the predetermined path and the second image is a right hand image of the radiation emitted by the surface of the article along the predetermined path, the first and second images together forming a stereoscopic image of the radiation emitted on the surface of the article along the predetermined path. 19. In a method of obtaining a three dimensional thermal image of constituent portions of an article both interior of and on the surface of the article,-the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal response on a surface thereof which is a thermal image of its constituent portions:

focusing a beam of radiant energy onto an incremental area of a first surface of the article to heat the area and, by the thermal conduction of the article,

to heat the portions of the article surrounding the heated area;

focusing each of two heat detection devices along an optical path and onto a separate one of two spaced incremental areas on a secondsurface of the article which is parallel to and opposite from the first surface, the detected incremental areas on the second surface being essentially opposite the heated incremental area on the first surface;

generating first and second electrical signals, each of which is in accordance with the thermal response detected at an incremental area by a separate one of the two heat detectors, the thermal response of an incremental area being a function of the thermal characteristics of internal and surface portions of the workpiece at and adjacent to the incremental area;

cyclically scanning the beam of radiation and the optical paths of the heat detectors across their associated article surfaces in a predetermined pattern and in unison with each other, to generate electrical signals which vary in accordance with the thermal responses of the scanned incremental areas;

forming first and second separate point images on a screen;

varying the brightness of each point image in accordance with the value of a separate one of the two electrical signals;

cyclically scanning each point image across a separate one of two areas on the screen in the predetermined pattern and in synchronism with the scan of the optical paths of the heat detectors across the second surface of the article in the predetermined pattern, to form on each area of the screen a separate visual representation of the thermal image of the constituent portions of the article as detected by a separate one of the two heat detection devices, and

rotating the article about an axis through an incremental amount, after each scan of the optical paths of the heat detectors through the predetermined pattern at least once, until the article has been rotated through a predetermined number of incremental amounts, and then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclically change the parallax between the optical paths of the heat detectors and the second surface of the article, so that, with the spaced relationship of the two detected incremental areas, the image on one area of the screen is a left hand thermal image of the constituent portions of the article while the image on the other area of the screen is a right hand thermal image of the constituent portions of the article which, taken together, form a stereoscopic thermal image of the constituent interior and surface portions of the article.

UNITED STATES PATENT OFFICE" CERTIFICATE OF CQRRECTION PatentNo. 3.805.073 m, April 16,1974

lnventods) Y, ,laya chandra-R. G. Watson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

El te "Assignee' should also include Bell Telephone Laboratorie i Incorporated, Murray Hill, New Jersey- Column 1, line 38, "C should read --o Column -8, line 63, "be" should read "by-- Column 8, line 48, "y-direction" should read --y-deflection--. Column 8, line 52, "x-direction" should read --x-deflection-. Column 8, line 56, "y-direction" should read --y-deflection--. Column 9, line 2]., "x-direction" should read --x-deflection--'. Column 9, line 22, "direction" should read -deflection-. Column 9, line 34, "througout" should read --throughout--. Column 12,- line 62, "non-contacting" should read --noncontacting--. Column 13,

line 35, "5" should read --4'--. Column 14, line 44, atters" should read --pattern--. Column 16, line 48, "portion' should read ortions", Column 19, lines 38-40, "successive incremental areas w ich overlie along the predetermined path" should be deleted. Column 19, line 48, after "plurality", insert Of successive incremental areas which overlie along the--. Column 7-, line 66, "of", second occurrence, should read or Signed and sealed this 15th day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

1. In a system for generating a stereoscopic thermal image of constituent parts of an article: means for heating the article; means, responsive to the heating experienced by the constituent parts of the article, as detected at pairs of discrete areas on a surface of the article, for generating a pair of signals which vary in accordance with the heating experienced by the constituent parts, and means, responsive to the pair of signals, for producing a stereoscopic thermal image of the constituent parts of the article.
 2. In a system for generating a stereoscopic thermal image of constituent parts of an article: means for scanning a beam of heat energy across a first surface of the article to heat the article; means, responsive to heat energy radiated by the heated constituent parts, at pairs of spaced discrete areas on a second surface of the Article which is opposite from the first surface of the article, for generating a pair of signals which vary in accordance with the heat energy radiated by the constitutent parts as the beam of heat energy scans the first surface, and means, responsive to the pair of signals, for producing a stereoscopic thermal image of the constituent parts of the articles.
 3. In a non-contacting apparatus for generating a stereoscopic thermal image of constituent parts of an article: means for transferring heat to the article; two heat detector means, each for detecting the thermal response of the constituent part of the article at a separate one of two discrete incremental areas of a surface of the article and for generating a signal in accordance therewith; means for scanning the heat detector means over the surface of the article to generate signals from the heat detector means which vary in accordance with the thermal responses of the scanned incremental areas; means for cyclically changing the parallax between the surface of the article and the incremental areas as detected by the heat detector means, and output indicator means for receiving the signals from the heat detector means and for producing a stereoscopic thermal image of the constituent parts of the article.
 4. In the noncontacting apparatus as set forth in claim 3, wherein: the means for transferring heat to the article includes means for directing infrared radiation onto a first surface of the article; the two heat detector means include two radiation pickup devices which are each responsive to detected infrared radiation from a separate one of two incremental areas on a second surface of the article to generate a signal which is representative of the radiation emitted by the article at the detected incremental areas on the second surface as the article is heated by the radiation on the first surface, and the means for scanning the heat detector means over the surface of the article includes optical scan means for cyclically scanning each of the pickup devices over the second surface in a predetermined pattern and in unison with each other.
 5. In a noncontacting apparatus as set forth in claim 5, wherein the means for cyclically changing the parallax includes: means, cyclically responsive after at least one cycle of the optical scan means, for rotating the article about an axis through an incremental amount until the article has been rotated through a predetermined number of incremental amounts in one direction of rotation, and for then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclically change the parallax between the second surface of the article and the scanned incremental areas thereon as detected by the pickup devices.
 6. In a noncontacting apparatus for detecting a characteristic of an article: means for transferring heat to an incremental area of a first surface of the article to heat the article; two heat detectors, each for detecting the thermal response of a separate one of two discrete incremental areas of a second surface of the article and for generating a signal in accordance therewith; means for cyclically scanning the heat transfer means and the heat detectors across their associated article surfaces in a predetermined pattern to heat successive scanned incremental areas on the first surface and to generate signals from the heat detectors which vary in accordance with the thermal responses of successive scanned incremental areas on the second surface; means, cyclically responsive after at least one cycle of the scanning means, for rotating the article about an axis through an incremental amount until the article has been rotated through a predetermined number of incremental amounts in one direction of rotation, and for then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclicalLy change the parallax between the second surface of the article and the scanned incremental areas thereon as detected by the heat detectors, and means for indicating the value of the signals generated by the heat detectors as the heat detectors are scanned across the second surface.
 7. In a noncontacting apparatus as set forth in claim 6, wherein: the means for transferring heat to an incremental area of the first surface to heat the article includes means for directing a beam of infrared radiation onto the area; the two heat detectors include two radiation pickup devices which are each focused along an optical path onto a separate one of the two incremental areas and which are each responsive to the detected infrared radiation from the detected incremental area on the second surface to generate a signal in accordance therewith, and the means for scanning the heat transfer means and the heat detectors across their associated surfaces includes optical scan means for cyclically scanning the beam of infrared radiation and the optical paths of the radiation pickup devices across their associated surfaces in the predetermined pattern.
 8. In the noncontacting apparatus as set forth in claim 7, wherein the means for indicating the value of the signals generated by the heat detectors includes: amplifier means, for receiving the signals from the pickup devices, and for generating output signals in accordance therewith, and visual indicator means, responsive to the signals from the amplifier means to generate two separate indications, each of which is representative of the radiation detected by a separate one of the pickup devices as the optical path thereof is scanned across the second surface of the article.
 9. In a scanning apparatus for inspecting a workpiece which exhibits a varying thermal heating patters in response to heating thereof: means for focusing a beam of radiation onto an incremental area of a first surface of the workpiece to heat the workpiece; two radiation sensing devices for receiving radiation and for generating an electrical signal in accordance with the intensity of the received radiation; means for focusing each radiation sensing device along an optical path and onto a separate one of two discrete incremental areas of a second surface of the workpiece, the second surface being opposite from and parallel to the first surface, and the incremental areas on the second surface being essentially opposite from the heated incremental area on the first surface, to generate an electrical signal with each sensing device in accordance with the radiation emitted by the incremental area on the second surface onto which the sensing device is focused, the radiation being in accordance with the thermal heating pattern exhibited by the article at that incremental area upon the heating of the article by the beam of radiation; means for cyclically scanning the beam of radiation and the optical path of each radiation sensing device across their associated workpiece surfaces in unison with each other to generate electrical signals with the sensing devices which vary in accordance with the radiation emitted by the successive scanned incremental areas on the second surface of the article; means, cyclically responsive after a predetermined number of cycles of the scanning means, for rotating the workpiece an incremental amount about an axis, to cyclically change the parallax between the thermal pattern exhibited by the workpiece on the second surface and the optical paths of the radiation sensing devices, and means for indicating the value of the electrical signals generated by the sensing devices as the optical paths of the sensing devices are scanned across the second surface of the workpiece.
 10. In a scanning apparatus as set forth in claim 9, wherein: each radiation sensing device is a radiometer; the scanning means includes a. first and second plane mirrors, each adapted for rotational oscilLation about an axis, the first plane mirror being positioned in the path of the beam of radiation, and the second plane mirror being positioned in the optical paths of the radiometers, to scan the beam of radiation and the optical paths of the radiometers across their associated surfaces in a first direction; b. means for cyclically oscillating the first and the second plane mirrors about their axes; c. third and fourth plane mirrors, each adapted for rotational oscillation about an axis, the third plane mirror being positioned in the path of the beam of radiation, and the fourth plane mirror being positioned in the optical paths of the radiometers, to scan the beam of radiation and the optical paths of the radiometers across their associated surfaces in a second direction which is orthogonal with respect to the first direction, and d. means for cyclically oscillating the third and the fourth plane mirros about their axes, and the means for indicating the value of the electrical signals generated by the radiometers includes a. first and second cathode ray tubes, each having an electron gun which receives an input which varies in accordance with the electrical signal from a separate one of the radiometers and which generates on a screen of the tube a raster which correspondingly varies in accordance with the electrical signal; b. first sweep means, synchronously coupled with the means for oscillating the first and second plane mirrors about their axes, for sweeping the raster across the screen of each cathode ray tube in a direction which corresponds with the first direction, and c. second sweep means, synchronously coupled with the means for oscillating the third and fourth plane mirrors about their axes, for sweeping the raster across the screen of each cathode ray tube in a direction which corresponds with the second direction to generate, in cooperation with the first sweep means, an image on the screen of each tube which is representation of the thermal heating pattern on the second surface of the workpiece as detected by a separate one of the radiometers as the optical path thereof is scanned across the second surface of the workpiece.
 11. In an apparatus for obtaining a three dimensional thermal image of constituent portions of an article both internal of the article and on the surfaces of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal response on a surface thereof which is a thermal image of its constituent portion: means for heating the article; path to generate from the thermal response detecting means signals means for detecting the thermal response of each of two spaced incremental areas on a surface of the article and for generating a separate signal in accordance with each detected thermal response; means for cyclically scanning the thermal response detecting means across the surface of the article in a predetermined means signals which vary in accordance with the thermal responses exhibited by the successive scanned incremental areas on the surface which lie along the predetermined path; means, cyclically responsive after at least one cycle of the scanning means, for rotating the article an incremental amount about an axis to vary the parallax between the surface of the article and the incremental areas as detected by the detecting means, and a visual display device, having a viewing screen, for receiving the separate signals from the detecting means and for generating on the screen thereof a separate visible image, in accordance with each signal, which is a visual representation of the thermal response exhibited on the surface of the article along the predetermined path as the thermal detecting means is scanned therealong, the separate visible images, with the spaced relationship of the two detected incremental areas, and with the cyclically varied parallax between the surface of the article and the incremental areas as detecTed by the detecting means, being left and right hand images of the thermal heating pattern exhibited by the article along the predetermined path, and together being a stereoscopic representation of the thermal image of the constituent portion of the article.
 12. In an apparatus for obtaining a three dimensional thermal image of constituent portions of an article both internal of the article and on the surfaces of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal pattern on a surface thereof which is a thermal image of its constituent internal and surface portions: means for focusing a beam of radiation onto an incremental area of a first surface of the article to heat the area and, by the thermal conduction of the article, to heat the article; two radiation pickup devices, each for receiving radiation and for generating a signal in accordance with the intensity thereof; means for focusing each radiation pickup device along an optical path and onto a separate one of two spaced incremental areas on a second surface of the article, the second surface being opposite from and parallel to the first surface, and the incremental areas on the second surface being essentially opposite from the heated incremental area on the first surface, to generate a signal with each radiation pickup device in accordance with the radiation emitted by the incremental area on the second surface onto which the pickup device is focused, the radiation being in accordance with the thermal heating pattern exhibited by the article at that incremental area upon the heating of the article by the beam of radiation; first scanning means for cyclically scanning the beam of radiation and the optical paths of the radiation pickup devices across their associated surfaces in a first direction and in unison with each other; second scanning means, simultaneously operative with the first scanning means, for cyclically scanning the beam of radiation and the optical paths of the radiation pickup devices across their associated surfaces in a second direction, to generate with the pickup devices signals which vary in accordance with the radiation emitted by the successive detected incremental areas on the second surface of the article as the article is simultaneously heated by the beam of radiation on the first surface; first and second video display devices, each having a screen for displaying video information thereon, and each for receiving at an input thereof a signal from a separate one of the radiation pickup devices and for generating on its screen a point image which varies in brightness in accordance with the value of the signal at its input, and therefore in accordance with the intensity of the radiation detected by the pickup device; first sweep means, synchronously coupled with the first scanning means, for cyclically sweeping the point image on each screen across the screen in a direction corresponding to the first direction, and in synchronism with the scan of the optical paths of the pickup devices across the second surface of the article in the first direction; second sweep means, synchronously coupled with the second scanning means, for cyclically sweeping the point image on each screen across the screen in a direction corresponding to the second direction, and in synchronism with the scan of the optical paths of the pickup devices across the second surface of the article in the second direction, to display on each screen, as the brightness of the point image thereon varies in accordance with the intensity of the radiation detected by a separate one of the radiation pickup devices, a visual representation of the thermal heating pattern on the second surface of the article as detected by a separate one of the pickup devices; means, cyclically responsive after at least one cycle of the second sweep means, for rotating the article about an axis through an incremental amount in one direCtion of rotation until the article has been rotated through a predetermined number of incremental amounts, and for then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclically change the parallax between the optical paths of the radiation pickup devices and the second surface of the article, so that, with the spaced relationship of the two detected incremental areas, one of the video display devices displays on its screen a left hand image of the thermal heating pattern on the second surface of the article, while the other video display device displays on its screen a right hand image of the thermal heating pattern on the second surface of the article, which, taken together, are a stereoscopic representation of the thermal image of the constituent internal and surface portions of the article.
 13. In an apparatus as set forth in claim 12, further including: means for filtering the beam of radiation to pass, to the first surface of the article, a beam of radiation which is transmissible with respect to one of the constituent portions of the article.
 14. In an apparatus as set forth in claim 12, wherein the first and second directions are orthogonal with respect to each other and the detected incremental areas lie along a line parallel to the first direction, and wherein: the radiation pickup devices are radiometers; the first and second video display devices are first and second cathode ray tubes, each having an electron gun for being controlled by the signal from a separate one of the radiometers and for generating, with an electron beam therefrom, the variable brightness point image on a screen of the tube; the first sweep means is a first deflection circuit for sweeping the electron beam from the gun in each cathode ray tube across the screen of the tube in a direction corresponding with the first direction; the second sweep means is a second deflection circuit for sweeping the electron beam from the gun in each cathode ray tube across the screen of the tube in a direction corresponding with the second direction; the first scanning means includes a. first and second plane mirrors adapted for oscillation about axes in synchronism with each other, the first plane mirror being in the path of the beam of radiation and the second plane mirror being in the optical paths of the radiometers, for scanning the beam of radiation and the optical paths of the radiometers across their associated article surfaces in the first direction, and b. first and second piezoelectric devices coupled to the first and second plane mirrors, respectively, for responding to a signal from the first deflection circuit to impart oscillating movement to the first and second plane mirrors in accordance therewith to scan the beam of radiation across the first surface of the article, and the optical paths of the radiometers across the second surface of the article in the first direction in synchronism with the sweep of the electron beams across the screens of the cathode ray tubes in the direction corresponding with the first direction; the second sweep means includes a. third and fourth plane mirrors adapted for oscillation about axes in synchronism with each other, the third plane mirror being in the path of the beam of radiation and the fourth plane mirror being in the optical paths of the radiometers, to scan the beam of radiation and the optical paths of the radiometers across their associated surfaces in the second direction, and b. third and fourth piezoelectric devices coupled to the third and fourth plane mirrors, respectively, for responding to a signal from the second deflection circuit to impart oscillating movement to the third and fourth plane mirrors in accordance therewith, to scan the beam of radiation across the first surface of the article and the optical paths of the radiometers across the second surface of the article in the second direction in synchronism with tHe sweep of the cathode ray tubes in the direction corresponding with the second direction, and the means for rotating the article about an axis through incremental amounts is a numberic controlled theta table coupled to the second deflection circuit, for rotating the article about an axis parallel to the second direction.
 15. In a method of producing a stereoscopic image indicative of thermal characteristics of constituent parts of an article: heating the article to produce a thermal response gradient across a surface of the article which varies in accordance with the thermal characteristics of the constituent parts; generating a pair of signals which vary in accordance with the thermal response gradient at a succession of pairs of discrete areas across the surface of the article, and producing, in accordance with the pair of signals, a stereoscopic image indicative of the thermal characteristics of the constituent parts of the article.
 16. In a method of generating a stereoscopic thermal image of the constituent parts of an article: heating the article; sequentially detecting the thermal response of the constituent parts of the article at each of a first plurality of successive incremental areas which lie along a predetermined path on a surface of successive incremental areas which lie along the predetermined path article; generating a first electrical signal which varies in accordance with the detected thermal response of the first plurality of successively detected incremental areas; sequentially detecting, simultaneously with detecting the thermal responses of the first plurality of incremental areas, the thermal response of the constituent parts of an article at each of a second plurality predetermined path on the surface of the article, the detected incremental area of the first plurality of incremental areas and the simultaneously detected incremental area of the second plurality of incremental areas being at a fixed spacing with respect to each other; generating a second electrical signal which varies in accordance with the detected thermal response of the second plurality of successively detected incremental areas; generating a first visual image, in accordance with the first electrical signal, representative of the thermal response detected at each of the first plurality of incremental areas, and generating a second visual image, in accordance with the second electrical signal, representative of the thermal response detected at each of the second plurality of incremental areas the first and second images beng left and right hand thermal images respectively which, taken together, form a stereoscopic thermal image of the constituent parts of the article.
 17. In a method of generating a stereoscopic thermal image of constituent parts of an article: heating the article; focusing each of two radiation pickup devices along an optical path at an angle with respect to a surface of the article, and onto a separate one of two spaced and discrete incremental areas on the surface to detect the thermal radiation emitted by the constituent parts thereat; generating a separate electrical signal in accordance with the radiation detected by each radiation pickup device; scanning the optical paths of the radiation pickup devices along a path on the surface of the article to detect with each pickup device the radiation emitted by successive scanned incremental areas to generate signals which vary in accordance with the radiation emission of the scanned incremental areas; cyclically changing the angle between the optical paths of the radiation pickup devices and the surface of the article, and generating in accordance with the value of the electrical signals, a stereoscopic thermal image of the constituent parts of the article.
 18. In a method as in claim 17, wherein: the scanning step includes cyclically scanning the optical paths of the pickup devices in unison with each otHer across the second surface of the article in a predetermined path to maintain the detected scanned incremental areas in a fixed spaced relationship with respect to each other; the indicating step includes generating first and second images, each of which is in accordance with the value of a separate one of the two electrical signals and is representative of the radiation emitted by the surface of the article along the predetermined path as detected by a separate one of the two pickup devices, and cyclically changing the angle between the optical paths of the pickup devices and the surface of the article includes cyclically rotating the article an incremental amount about an axis, in response to at least one scan of the optical paths of the pickup devices across the surface of the article, so that with the spaced relationship of the detected incremental areas the first image is a left hand image of the radiation emitted by the surface of the article along the predetermined path and the second image is a right hand image of the radiation emitted by the surface of the article along the predetermined path, the first and second images together forming a stereoscopic image of the radiation emitted on the surface of the article along the predetermined path.
 19. In a method of obtaining a three dimensional thermal image of constituent portions of an article both interior of and on the surface of the article, the portions having differing thermal heating characteristics, and the article, when heated, exhibiting a thermal response on a surface thereof which is a thermal image of its constituent portions: focusing a beam of radiant energy onto an incremental area of a first surface of the article to heat the area and, by the thermal conduction of the article, to heat the portions of the article surrounding the heated area; focusing each of two heat detection devices along an optical path and onto a separate one of two spaced incremental areas on a second surface of the article which is parallel to and opposite from the first surface, the detected incremental areas on the second surface being essentially opposite the heated incremental area on the first surface; generating first and second electrical signals, each of which is in accordance with the thermal response detected at an incremental area by a separate one of the two heat detectors, the thermal response of an incremental area being a function of the thermal characteristics of internal and surface portions of the workpiece at and adjacent to the incremental area; cyclically scanning the beam of radiation and the optical paths of the heat detectors across their associated article surfaces in a predetermined pattern and in unison with each other, to generate electrical signals which vary in accordance with the thermal responses of the scanned incremental areas; forming first and second separate point images on a screen; varying the brightness of each point image in accordance with the value of a separate one of the two electrical signals; cyclically scanning each point image across a separate one of two areas on the screen in the predetermined pattern and in synchronism with the scan of the optical paths of the heat detectors across the second surface of the article in the predetermined pattern, to form on each area of the screen a separate visual representation of the thermal image of the constituent portions of the article as detected by a separate one of the two heat detection devices, and rotating the article about an axis through an incremental amount, after each scan of the optical paths of the heat detectors through the predetermined pattern at least once, until the article has been rotated through a predetermined number of incremental amounts, and then rotating the article through the predetermined number of incremental amounts in the opposite direction of rotation, to cyclically change the parallax between the optical paths of the heat detectors and the second surface of the article, so that, with the spaced relationship of the two detected incremental areas, the image on one area of the screen is a left hand thermal image of the constituent portions of the article while the image on the other area of the screen is a right hand thermal image of the constituent portions of the article which, taken together, form a stereoscopic thermal image of the constituent interior and surface portions of the article. 